The present invention relates to a method for producing dual zone porous materials having an external zone bearing first silyl groups immobilized on the external surfaces of the porous material and internal zone having second silyl groups immobilized on the internal surfaces of the porous material and the dual zone materials so produced. More particularly, it relates to a method for contracting a porous hydroxyl-bearing support simultaneously with two different organosilanes which differentially react with the external and internal hydroxyl groups on the porous support and production thereby of a dual zone material.
In copending applications Ser. No. 154,754 and Ser. No. 598,120, there are disclosed dual surface (more properly termed dual zone) porous materials made by treating a porous hydroxyl-bearing support, such as porous silica, alumina, zirconia, etc., with substoichiometric amounts of an ultrafast silylating agent. This agent is chosen from those which are so reactive that the resulting surface group is immobilized in the external zone of the porous support before the agent has had time to migrate deeply into the porous internal zone. A subsequent silylation reaction can be employed to convert residual hydroxyl groups, which reside predominantly in the internal zone, to a second immobilized group of another type. See also, Williams & Tangney, Silanes, Surfaces & Interfaces, D. E. Leyden, ed., Gordon & Breach Publisher, 1986, P. 471 ff.
In copending application Ser. No. 598,120, the disclosed ultrafast silylating agents are reactive silane intermediates. In patent application Ser. No. 154,754 the ultrafast silylating agents are silanes having "leaving groups" such as (i) substituted amides, (ii) substituted amines, or (iii) thioethers. It is believed that these facile leaving groups lower the activation energy required for reaction with surface hydroxyl groups and thus enhance the extent to which the silane can be captured by covalent bond formation in the external zone of the porous material, that is, captured early during its diffusion path into said material.
As stated in copending applications Ser. No. 154,754 and Ser. No. 598,120, traditional silylation reactions are generally not fast enough to permit preferential silylation of the external surface of the porous support. "Traditional silylation" is described in Plueddemann, Encyclopedia of Chemical Technology, 3rd ed., Vol. 20, page 962 et seq. Plueddemnn states that silylation is the displacement of active hydrogen from an organic molecule by silyl groups where "The active hydrogen is usually OH, NH, or SH, and the silylating agent is usually a trimethylsilyl halide or a nitrogen-functional compound. A mixture of silylating agents may be used; a mixture of trimethychlorosilane and hexamethyldisilazane is more reactive than either reagent alone, and the by-products combine to form neutral ammonium chloride".
Neither consecutive nor simultaneous treatment of porous supports with two such traditional silylating agents has produced a dual zone porous material of the type described in parent application Ser. No. 654,754. For example, Abbott in U.S. Pat. No. 4,298,500 discloses sequentially treating a porous silica gel with an organosilane reagent to form a "first residue" and, then, an organosilane-containing diol, diol precursor or amide to form a "second residue". However, the resulting product is a mixed phase composition which shows negligible dual zone characteristics.
Likewise, Marshall et al. in "Synthesis of LC Reversed Phases of Higher Efficiency by Initial Partial Deactivation of the Silica Surface". Journal of Chromatography Science, Vol. 22, June 1984 pp. 217-220, disclose first treating silica with a small amount of end-capping reagent (such as trimethylchlorosilane) followed by exhaustive octadecylation. Again the result is a homogenous distribution of surface bound molecules.
In terms of simultaneous treatment with a mixture of reactants reference is made to the Plueddemann publication mentioned above and the M. L. Hunnicutt and J. M. Harris, "Reactivity of Organosilane Reagents on Microparticulate Silica", Anal. Chem., Vol. 58, Apr. 1986, pp. 748-752. Hunnicutt and Harris discuss the results of competitive surface reactions between binary organosilane mixtures and silica gel. The organosilane mixtures used include mixtures of two haloalkylsilanes such as (1-bromomethyl)dimethylmonochlorosilane, (1-chloromethyl)dimethylmonochlorosilane, or (3-chloropropyl)dimethylmonochlorosilane, as well as mixtures of a haloalkylsilane with an alkysilane such as trimethylchlorosilane (TMCS) or hexamethyldisilazane (HMDS). In a number of instances a catalyst such as pyridine was added to the silica slurry prior to silane addition for base catalyzed reactions. Hunnicutt and Harris showed that their reaction did not display pore diffusion control. Thus they could not have produced dual zone materials (DZMs) with respect to differential distribution of their chosen immobilized groups. This outcome is believed to be due to several factors. Most importantly, mixtures of chlorosilanes of the type used by Hunnicutt and Harris do not react with sufficient speed and differentiality even when the reaction is catalyzed with pyridine.
Furthermore, the reaction conditions were not adjusted so as to produce DZMs even from the point of view of selective capture of both chlorosilanes together in the external zone. Firstly, the solvent they used was chloroform which is highly polar and is known to be a proton donor in hydrogen-bonded complexes. Such solvents have been found to reduce pore diffusion control, probably by sequestering the surface reactive sites (silanol) and thus slowing down the reaction rate. Protic solvents such as ethanol are even more deleterious since the halosilane is solvolyzed and transformed into the less reactive ethoxysilane. Secondly, the rate of silane addition to the silica slurry was excessively fast at about 0.3 ; molecules/nM.sup.2 /minute. Accordingly, individual silica particles would be subjected to unusual doses of silane and the resultant particle-to-particle heterogeneity would overcome any intraparticle inhomogeneity (dual zone structure) that might otherwise occur. Accordingly, even though Hunnicutt and Harris conducted what could be described as a mixed halosilane reaction, Hunnicutt and Harris do not teach one of ordinary skill in the art how to produce dual zone materials by means of such reaction mechanisms.
And yet, it is known to be desirable to produce dual zone porous materials having silyl groups of one type predominantly on the external surface and silyl groups of another type predominantly on the internal surface in order to provide on the external and internal surfaces differentially selective adsorbents, for example, for specific chromatographic and catalytic applications. It would also be desirable to use a mixture of organosilanes because of the ease and lower cost involved. To date, however, it has not been possible to do so.
Accordingly, the need remains for a method for simultaneously contacting a porous hydroxyl-bearing support with a mixture of organosilanes in the production of dual zone porous materials.